This invention relates to computed tomography (CT) systems and specifically to a CT system having an x-ray tube whose focal spot may be controllably translated along the plane of the CT gantry rotation.
In a computed tomography system, an x-ray source is collimated to form a fan beam with a defined fan beam angle. The fan beam is oriented to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane", and to be transmitted through an imaged object to an x-ray detector array oriented within the imaging plane. The detector array is comprised of detector elements, centered on a "pitch", each of which measure the intensity of transmitted radiation along a beam projected from the x-ray source to the particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object. The center of a beam and its intensity measurement may be identified to a ray described by the line joining the center spot of the x-ray source and the center of the detector element.
The x-ray source and detector array may be rotated on a gantry within the imaging plane and around the imaged object so that the angle at which the fan beam intersects the imaged object may be changed. At each gantry angle, a projection is acquired comprised of the intensity signals from each of detector elements. The gantry is then rotated to a new angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set.
The acquired tomographic projection sets are typically stored in numerical form for computer processing to "reconstruct" a slice image according to reconstruction algorithms known in the art. A projection set of fan beam projections may be reconstructed directly into an image by means of fan beam reconstruction techniques, or the intensity data of the projections may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. The reconstructed tomographic images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
The spatial resolution of the reconstructed CT image is dependant, in part, on the width of each x-ray beam at the center of the imaged object. This beam width is determined primarily by the source width, the size of the focal spot of the x-ray tube, and aperture of the detector element, and varies with distance from the source and detector. The averaging effect of a generally rectangular beam of width a, bandlimits the received image to a spatial frequencies of 1/a and less.
The beam spacing, defined near the center of the imaged object and determined by the detector pitch, controls the spatial sampling frequency of the CT system. Given the spatial bandlimit of 1/a, above, the sampling frequency must be approximately 2/a, per the Nyquist sampling thereon, to avoid aliasing effects in the reconstructed image. The elimination of aliasing therefore requires that the beam be sampled or read at distances separated by one half the beam width. Ordinarily, the beam width is optimized to be substantially equal to the beam spacing and therefore sampling is ideally performed no less than twice per beam spacing. This sampling will henceforth be referred to as double sampling.
A conceptually simple way to accomplish double sampling is to shift the detector elements one half of their pitch after a first sample and to take a second sample. In this way each beam is sampled twice in its width (and spacing). Nevertheless, the mechanical problems incident to rapidly and precisely moving the detector elements by one half their pitch (typically on the order of 1 mm) make this approach impractical. Rather, two other method are used:
The first method is to offset the detector elements in the plane of gantry rotation one quarter of the detector's pitch with respect to the gantry's axis of rotation. Beams projected through the imaged object at angles separated by 180.degree. will be offset from each other by one half of the detector pitch and hence by one half the beam spacing for an optimized beam.
Although this method is relatively simple, it requires a full 360.degree. of scanning and hence is not usable with reduced angle scanning techniques that acquire less than 360.degree. of scanned data. Further, for this method to work properly, the imaged object must not move in between the acquisition of data for each offset beam. The length of time needed for the gantry to rotate 180.degree. may be on the order of a second or more and hence motion of the imaged object is inevitable especially for organs such as the heart.
The second method of performing double sampling of each beam is to wobble the x-ray source by an amount that will shift each beam by one half its spacing. The wobbling is generally within the plane of rotation of the gantry and along the tangent to the gantry rotation. Wobbling of the x-ray source is easily accomplished electronically without mechanical motion of the x-ray tube. In an x-ray tube, an electron beam is accelerated against an anode at a focal spot to produce x-ray radiation emanating from the focal spot. The focal spot may be moved on the surface of the anode by the use of deflection coils or plates within the x-ray tube which deflect the electron beam either by the creation of a local magnetic or electrostatic field as is well understood in the art.
Double sampling may be performed by taking a first set of data with the x-ray spot in a first position on a first 360.degree. scan; and taking a second set of data with the focal spot shifted to a second position on a second 360.degree. scan. Preferably, however, to avoid motion problems between adjacent samples, the x-ray beam is rapidly shifted from one position to the other between each projection.
The rate at which the x-ray beam can be wobbled is limited by the acquisition time of the detector elements. This acquisition time, in turn, is dependant primarily on two factors: the decay time of the detector signal after stimulation by an x-ray beam and the desired signal-to-noise ratio of the projection data. The decay time is a function of the detector design. The signal-to-noise ratio is principally a function of the detector integration time, that is, how long the detector is allowed to collect x-ray energy.
The acquisition time restricts the rate at which the x-ray beam may be wobbled between focal spots to produce offset projections. Accordingly, and as will be explained in more detail below, the wobbled projections will be not only shifted by one half of the beam spacing with the movement of the focal spot of the x-ray tube (as desired) but also rotated from the ideal acquisition point by gantry rotation during the acquisition time. Therefore, one drawback to wobbling the x-ray source is that the projection data is not collected at the optimal positions for image reconstruction. Such misalignment between the data of a projection and its wobbled image degrades the resolution of the reconstructed image at points removed from its center.